There are a number of radionuclide emission tomography cameras utilizing scintillation material which emits light in response to absorbed radiation emanating from an object to be studied. The scintillations are detected by an array of photodetectors.
Presently, the photodetectors are optically coupled to the scintillation material by a single, integral light pipe which has an area corresponding to the combined area of an entire array of photodetectors. In Genna et al., U.S. Pat. No. 4,095,107, for example, light pipe 45 extends between photodetector array 47 and scintillation crystal 43, FIG. 4.
For scintillation material which is curvilinear, such as the arcuately shaped crystal of the above patent or the annular single crystal of Genna et al., U.S. Pat. No. 4,584,478, fashioning a light pipe having an inner face which matches the curvature of the curvilinear scintillation material and an outer face which matches the photodetectors is difficult and expensive. The need for such a light pipe has hampered efforts to develop cost-effective scintillation cameras which utilize curvilinear scintillation material. Further, it is difficult to correctly position and maintain the alignment of photodetectors relative to the curved scintillation material.
An additional problem is posed by curved scintillation material. The location of a radionuclide source within an object to be imaged is determined from gamma ray trajectories, whose collimated direction is in turn determined from the light emissions they produce in the scintillation material. Resolution by the photodetector array of the gamma ray trajectories depends on the proportion of light emission that is collected by the photodetector nearest the emission and photodetectors adjacent to it.
The distance of the edges of the photodetector to the inner face of the scintillation material largely determines the distribution of light collected by the photodetectors and therefore controls resolution. However, for an annular or arcuately-shaped scintillation crystal, the distance from the photodetector edges to the face of the scintillation material, hereinafter referred to as the separation distance, is different when viewed azimuthally as compared to axially. The separation distance seen by light travelling in the azimuthal plane is determined by the photodetector edges perpendicular to the azimuthal plane, and these edges are a uniform distance from the scintillation material. However, light travelling along one plane parallel to the axis of rotation will encounter a different separation distance than that encountered by light travelling along another axial plane which intersects the detector face at a different azimuthal location.
In three-dimensional radionuclide reconstruction using single photon emission computed tomography employing a stationary annular crystal with rotating collimator, as shown in U.S. Pat. No. 4,584,478, or a tomographic system in which the camera's detector and collimator rotate in unison, as shown in U.S. Pat. No. 4,095,107, one-dimensional resolution in the azimuthal direction determines, through image reconstruction, two-dimensional resolution in an azimuthal plane normal to the axis of rotation. This two-dimensional resolution is not affected by resolution in the axial direction. Higher resolution may be desired in the azimuthal direction, for example, if diagnoses are made from images obtained from azimuthal slices, normal to the axis of rotation, of three-dimensional reconstructed images.
Within limits, the average resolution of a crystal detector/photodetector array is improved as the photodetectors are moved closer to the scintillation material. However, the resolution, as a function of scintillation position relative to the center of the photodetectors, becomes increasingly nonuniform as the distance decreases. Therefore, a compromise between average resolution and nonuniformity must be reached.
Moreover, the difference in effective separation distance of the photodetector edges and the different criteria governing the optimization of resolution characteristics generate different optimal separation distances for the azimuthal and axial directions. A compromise must therefore also be reached in setting the final separation distance of the photodetector from the scintillation material.